Apraxia of speech (AOS) is a motor speech disorder that poses significant obstacles to a person's ability to communicate and take part in everyday life. Current theories of AOS agree that the impairment affects the speech motor planning stage, where linguistic representations are transformed into speech movements, but they disagree on the specific nature of the breakdown at this processing level (Ziegler, 2002). A more detailed understanding of this impairment is essential for developing targeted, effective treatment approaches and for identifying the appropriate candidates for these treatments. The study of AOS is complicated by the fact that this disorder rarely occurs in isolation but is commonly accompanied by various degrees of aphasia (a language impairment) and/or dysarthria (a neuromuscular impairment of speech motor control). In addition, the behavioral similarities of AOS and its closest clinical neighbor, aphasia with phonemic paraphasias, undermine the usefulness of traditional methods, such as perceptual error analysis, in the study of both disorders. The purpose of the present proposal is to test two competing hypotheses about the specific nature of the speech motor planning impairment in AOS. The project is formulated in the context of a well-established theoretical framework of speech production and it combines psycholinguistic reaction time paradigms with a cognitive neuropsychological approach. This innovative approach to the study of AOS is promising for the following reasons. First, psycholinguistic reaction time methods have had an important role in improving our understanding of speech motor planning in unimpaired speakers, but they have rarely been applied to the study of people with neurologic impairments. Second, this combined approach circumvents the problem of behavioral similarity of error patterns in different disorders by focusing on the onset latencies of the same behavior under different speech motor planning conditions. A few initial studies have suggested different reaction time patterns between people with AOS and people with aphasia without AOS, supporting the potential utility of this systematic line of research in regard to differential diagnosis. The successful completion of this research project will yield novel information about the specific motor planning impairment in AOS. To date, different specific hypotheses about the impairment in AOS have been proposed but they have not been directly tested against each other with the same methods in a systematic succession of experiments. In addition, the proposed research project will test the specificity of the speech motor planning impairment in speakers with AOS by comparing the findings from speakers with AOS to those from speakers with aphasia. These findings will form a critical, theoretically-motivated basis for improving diagnostic tools and treatment protocols for people with AOS and aphasia, thus enhancing clinical decision- making. Such translational and clinical research aimed at developing sensitive and specific diagnostic tools and improving treatment protocols is the ultimate long-term objective of this candidate's research program.